Printing machines using a doctor blade associated with a ductor roller permit adjustment of the ink picked by the ductor roller in zones along the length of the ductor roller. The doctor blade is flexible, and by suitably controlling adjustment screws bearing against, or connected to the doctor blade, the nip or gap between the doctor blade and the ductor roller can be adjusted so that the thickness of the ink film on the ductor roller being picked up is controllable. This control is desirable since zones carrying a greater quantity or density of printing material require a greater quantity of ink than zones which are essentially clear or free of printing. Offset printing machines, thus, can provide improved quality output if the ink distribution along the circumference of the inking roller is non-uniform.
Apparatus has been proposed to determine the brightness or darkness of models, patterns or samples, or of printing plates carrying more or less information content thereon, transversely to the transport direction thereof by deriving, photoelectrically, output signals representative of the density distribution of printed material over the pattern or printing plate. The output signals can then be used directly to indicate manual adjustment of doctor blade adjustment screws, can be stored, displayed, or connected in a real-time apparatus to servo motors controlling the adjustment of doctor blade control screws.
It has been proposed to adjust the position of a doctor blade in the ink fountain system of a rotary offset printing machine by evaluating the areal coverage of a sample or pattern film, and deriving adjustment signals representative thereof to permit control and adjustment of the zone screws controlling the relative position of the doctor blade and the ductor roller (see, for example, German Patent Disclosure Document DE-OS No. 17 61 333). For example, and as described in this reference, positive film, negative film, or display patterns can be either illuminated by shining light through film, or reflecting light from the pattern or model, and deriving representative output signals; likewise, the printing plates themselves or paper patterns can be used to obtain an individual measurement to adjust the respective zone adjustment screws of the doctor blade associated with the ink ductor roller.
The measuring zone covered by the evaluation apparatus should have the same width as the control zone of the adjustment of the doctor blade in the printing machine, in which the utilization signals are to be used to control the adjustment screws of the doctor blade. It has already been proposed (see German Patent Disclosure Document DE-OS No. 26 18 387) to provide a film scanning apparatus in which a transducing apparatus is provided which has a width matching the zones which are identical to the width of the zone controlled by the adjustment screws of the printing machine, in which the pattern being analyzed is to be printed. This arrangement requires a scanning apparatus which is specific to a certain zone width, matched to the adjustment width of the printing machine, and thus cannot be used with various types of printing machines having different widths of adjustment zones, nor with models having different widths of the zones of matter to be printed.